Device for determining the abnormal degree of deterioration of a catalyst

ABSTRACT

A device for determining the abnormal degree of deterioration of a catalyst of a catalytic converter arranged in an internal combustion engine exhaust system is disclosed. The device includes first and second air-fuel ratio sensors located upstream and downstream of the catalyst. The device determines a first purification ability based on readings of the sensors during a warmup stage of the catalyst, and a second purification ability after the warmup is completed. An overall purification ability of the catalyst is determined by combining the purification ability of the catalyst during warmup and the purification ability of the catalyst after warmup, where the weight of the purification ability during warmup is less than the purification ability after warmup in the overall purification ability. If the overall purification ability of the catalyst is less than a certain amount, then the device determines that the catalyst has deteriorated.

This application is a division of prior application Ser. No. 08/632,375,filed Apr. 10, 1996 now U.S. Pat. No. 5,765,370.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for determining the abnormaldegree of deterioration of a catalyst.

2. Description of the Related Art

A catalyst is usually arranged in an exhaust passage to purify theexhaust gas. Once the catalyst deteriorates excessively, the catalystcannot purify the exhaust gas sufficiently. Accordingly, it is necessaryto determine the abnormal degree of deterioration of the catalyst, toinform the driver about this, and to urge the driver to exchange thecatalyst for a new one.

The exchange is costly and requires much time. Accordingly, it isnecessary to determine the abnormal degree of deterioration of acatalyst precisely. In a usual device for determining the abnormaldegree of deterioration of a catalyst, the current purification abilityof a catalyst is detected after the catalyst has been activated and,thereafter, the current purification ability is compared with thethreshold which corresponds to the abnormal degree of deteriorationthereof. In a catalyst, only a purification ability before the catalysthas been activated can drop excessively. In this case, the usual devicedetermines that the degree of deterioration of the catalyst is normal.Therefore, the catalyst is not exchanged and thus the exhaust gas is notcompletely purified before it has been activated.

To solve this problem, Japanese Unexamined Patent Publication No.5-248227 discloses a device for determining the abnormal degree ofdeterioration of a catalyst. The device corrects the threshold whichcorresponds to the abnormal degree of deterioration of a catalyst,according to the degree of activation thereof, i.e., the temperaturethereof, detects a current purification ability thereof before and afterthe catalyst has been activated, compares the current purificationability with the corrected threshold, and determines if the degree ofdeterioration thereof is abnormal.

However, a purification ability of a catalyst before the catalyst hasbeen activated is particularly unstable, because it varies largelyaccording to not only the temperature thereof but also the harmfulmaterials content of the exhaust gas and the like. Accordingly, if thedevice merely compares the current purification ability with thecorrected threshold and determines the degree of deterioration of thecatalyst, the device can mistake the determination.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a device fordetermining the abnormal degree of deterioration of a catalyst andcapable of determining accurately if the degree of deterioration thereofis abnormal.

According to the present invention, there is provided a device fordetermining the abnormal degree of deterioration of catalyst of acatalytic converter arranged in an internal combustion engine exhaustsystem, comprising: purification ability grasping means for grasping acurrent purification ability of the catalyst; temperature grasping meansfor grasping a current temperature of the catalyst; thresholddetermining means for determining a threshold of the purificationability which corresponds to the abnormal degree of deterioration of thecatalyst, in accordance with the current temperature of the catalyst;appraisal value determining means for determining an appraisal value, inaccordance with a difference between the threshold and the currentpurification ability; integration value calculating means forcalculating an integration value of the appraisal value; and abnormalitydetermining means for determining that the degree of deterioration ofthe catalyst is abnormal when the integration value exceeds apredetermined value.

According to the present invention, there is provided another device fordetermining the abnormal degree of deterioration of catalyst of acatalytic converter arranged in an internal combustion engine exhaustsystem, comprising: first purification ability grasping means forgrasping a first purification ability of the catalyst in the completeactivation condition of said catalyst; second purification abilitygrasping means for grasping a second purification ability of thecatalyst in an incomplete activation condition of said catalyst; overallpurification ability calculating means for calculating an overallpurification ability of the catalyst such that the first purificationability given a first weight is added to the second purification abilitygiven a second weight which is less than the first weight; and abnormaldetermining means for determining if the degree of deterioration of thecatalyst is abnormal by the comparison between the overall purificationability and a predetermined threshold thereof.

According to the present invention, there is provided a further anotherdevice for determining the abnormal degree of deterioration of catalystof a catalytic converter arranged in an internal combustion engineexhaust system, comprising: first purification ability grasping meansfor grasping a first purification ability of the catalyst in completeactivation condition of said catalyst; a second purification abilitygrasping means for grasping a second purification ability of thecatalyst in incomplete activation condition of the catalyst; firstprovisional determining means for determining provisionality if thedegree of deterioration of the catalyst is abnormal by the comparisonbetween the first purification ability and a first predeterminedthreshold thereof; second provisional determining means for determiningprovisionality if the degree of deterioration of the catalyst isabnormal by the comparison between the second purification ability and asecond predetermined threshold thereof; and main determining means fordetermining if the degree of deterioration of the catalyst is abnormalby the results of the first and second provisional determining means.

The present invention will be more fully understood from the descriptionof preferred embodiments of the invention set forth below, together withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a section view of a part of an internal combustion engineexhaust system with a device for determining the abnormal degree ofdeterioration of a catalyst;

FIG. 2 is a part of a first routine for determining the abnormal degreeof deterioration of a catalyst;

FIG. 3 is the remainder of the first routine;

FIG. 4 is a map for determining an amount of basic reaction heat used inthe first routine;

FIG. 5 is a map for determining an amount of basic radiation heat usedin the first routine;

FIG. 6 is a routine for calculating a first and second integration valueused in the first routine;

FIG. 7 is a map for determining a threshold used in the first routine;

FIG. 8 is a map for determining an appraisal value used in the firstroutine;

FIG. 9 is another map for determining an appraisal value used in thefirst routine;

FIG. 10 is a second routine for determining the abnormal degree ofdeterioration of a catalyst;

FIG. 11 is a part of a third routine for determining the abnormal degreeof deterioration of a catalyst;

FIG. 12 is the remainder of the third routine;

FIG. 13 is a map for directly determining an appraisal value;

FIG. 14 is a sectional view of a part of an internal combustion engineexhaust system with another device for determining the abnormal degreeof deterioration of a catalyst;

FIG. 15 is a part of a fourth routine for determining the abnormaldegree of deterioration of a catalyst;

FIG. 16 is the remainder of the fourth routine;

FIG. 17 is a routine for calculating the purification ability of thecatalyst;

FIG. 18 is a map for determining a coefficient used in the routine ofFIG. 17;

FIG. 19 is a fifth routine for determining the abnormal degree ofdeterioration of a catalyst;

FIG. 20 is a part of a sixth routine for determining the abnormal degreeof deterioration of a catalyst;

FIG. 21 is the remainder of the sixth routine;

FIG. 22 is a sectional view of a part of an internal combustion engineexhaust system with a further another device for determining theabnormal degree of deterioration of a catalyst;

FIG. 23 is another routine for calculating the purification ability ofthe catalyst; and

FIG. 24 is a map for determining HC concentration used in the routine ofFIG. 23.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view of a part of an internal combustion engineexhaust system with a device for determining the abnormal degree ofdeterioration of a catalyst according to the present invention.Referring to FIG. 1, reference numeral 1 designates a three-waycatalytic converter which purifies the exhaust gas. The three-waycatalytic converter has an O₂ storage ability such that it absorbs andstores excess oxygen existing in the exhaust gas when the air-fuel ratioin the exhaust gas is on the lean side, and it releases oxygen when theair-fuel ratio in the exhaust gas is on the rich side. Therefore, theair-fuel ratio in the exhaust gas becomes almost stoichiometric so thatthe three-way catalytic converter 1 can purify the exhaust gassufficiently. The upstream side of the three-way catalytic converter 1is connected to the engine body (not shown). On the other hand, thedownstream side thereof is opened to the atmosphere, via a muffler (notshown). Reference numeral 2 designates a catalytic carrier which carriesthe catalyst. Reference numeral 3 designates a first air-fuel ratiosensor which detects an air-fuel ratio in the exhaust gas flowing intothe catalytic carrier 2. Reference numeral 4 designates a secondair-fuel ratio sensor which detects an air-fuel ratio in the exhaust gasflowing out from the catalytic carrier 2. The first and second air-fuelratio sensors 3, 4 produce an output voltage which is proportional tothe air-fuel ratio in the exhaust gas.

The three-way catalytic converter 1 gradually deteriorates with the usethereof. Once the degree of deterioration of the three-way catalyticconverter 1 becomes abnormal, the purification ability thereof becomesvery low so that the exchange thereof is necessary. Reference numeral 30is a device which determines the time of such exchange, i.e., theabnormal degree of deterioration of the catalyst.

The device 30 is an electronic control unit (ECU). The ECU 30 isconstructed as a digital computer and includes a ROM (read only memory)32, a RAM (random access memory) 33, a CPU (microprocessor, etc.) 34, aninput port 35, and an output port 36, which are interconnected by abidirectional bus 31. The output voltages of the first and secondair-fuel ratio sensors 3, 4 are input into the input port 35 via an ADconverters 37a, 37b, respectively. An engine speed sensor 23, whichproduces an output pulse representing the engine speed, is connected tothe input port 35. An air flow meter 24 produces an output voltage whichis proportional to the amount of air fed into the engine cylinder, andthis output voltage is input into the input port 35 via an AD converter37c. A first temperature sensor 25 produces an output voltage which isproportional to the temperature of the engine cooling water, and thisoutput voltage is input into the input port 35 via an AD converter 37d.A second temperature sensor 26 produces an output voltage which isproportional to the temperature of the atmosphere, and this outputvoltage is input into the input port 35 via an AD converter 37e. Theoutput port 36 is connected through a drive circuit 38 to an alarm lamp10 showing that the degree of deterioration of the three-way catalyticconverter 1 becomes abnormal. In the engine, an amount of injected fuelis controlled such that an air-fuel ratio in the mixture becomes almoststoichiometric by means of the first and second air-fuel ratio sensors3, 4. The fuel injection control is a conventional air-fuel ratiofeed-back control.

FIGS. 2 and 3 show a first routine for determining the abnormal degreeof deterioration of the three-way catalytic converter 1. The firstroutine is started simultaneously with the engine starting and isrepeated at every predetermined period. First, at step 101, a currentamount of intake air Q_(n) !, a current engine speed Ne_(n) !, a currenttemperature of the engine cooling water Thw_(n) !, and a currenttemperature of the atmosphere Ta_(n) ! are detected by theabove-mentioned sensors. Next, at step 102, it is determined if a flagF! is 2!. The flag F! is reset to 0! when the engine is stopped.Accordingly, the result at step 102 is negative and the routine goes tostep 103. At step 103, it is determined if a flag F! is 1!. The resultis negative similarly and the routine goes to step 104.

At step 104, it is determined if a difference between the temperature ofthe engine cooling water Thw_(n) ! and the temperature of the atmosphereTa_(n) ! is larger than a predetermined value A!. When the result isaffirmative, the engine has been started again immediately after it wasstopped so that the routine goes to step 105 and the flag F! is made 1!and the routine is stopped. On the other hand, when the result at step104 is negative, the routine goes to step 106 and the flag F! is made2!. Thereafter, the routine goes to step 107.

At step 107, the temperature of the atmosphere Ta_(n) ! is made anassumed temperature of the catalyst at the last time Tc_(n-1) !. At step108, an amount of basic reaction heat BTr_(n) ! at this time isdetermined by a map shown in FIG. 4, on the basis of the assumedtemperature of the catalyst at least time Tc_(n-1) !. An amount of basicreaction heat is an amount of heat which is generated by thepurification of the exhaust gas in a current temperature of thecatalyst, i.e., a current degree of activation of the catalyst.Accordingly, the higher the temperature of the catalyst becomes, thelarger an amount of basic reaction heat is set on the map shown in FIG.4. The amount of basic reaction heat varies according to not only thetemperature of the catalyst, but also the current purification abilityof the catalyst. Accordingly, a similar map is provided in everypurification ability of the catalyst and a map used at step 108 isselected from these maps on the basis of the purification ability at thelast time Lr_(n-1) ! which is explained below in detail.

Next, at step 109, an amount of basic radiation heat at this timeBTo_(n) ! is determined by a map shown in FIG. 5, on the basis of theassumed temperature of the catalyst at the last time Tc_(n-1) !. Theamount of basic radiation heat is an amount of heat which radiates fromthe catalyst. Accordingly, the higher the temperature of the catalystbecomes, the larger an amount of basic radiation heat is set on the mapshown in FIG. 5.

At step 110, an amount of exhaust gas flowing into the catalytic carrier2 is calculated by an expression (1), as an amount of calculated intakeair Qsm_(n) !.

    Qsm.sub.n =Qsm.sub.n-1 +(Q.sub.n -Qsm.sub.n-1)*Ne.sub.n /N (1)

The expression (1) represents smoothing process of an amount of intakeair. In the expression (1), Q_(n) ! is an amount of measured intake airat this time. Qsm_(n-1) ! is an amount of calculated intake air at thelast time and is set as the usual amount of idle intake air, as aninitial value immediately after the engine is started. Of course, theinitial value can take account of the variation of idle intake airaccording to the temperature of the engine cooling water Thw!. Ne_(n) !is the engine speed. N! is a predetermined value. The premise ofexpression (1) is that the lower the current engine speed is, thesmaller the absolute value of the varying amount of intake air is.

Next, the routine goes to step 111 and a varying value of thetemperature of the catalyst at this time dTc_(n) ! is calculated by anexpression (2).

    dTc.sub.n =K1*(K2*Qsm.sub.n +K3*BTr.sub.n -K4*BTo.sub.n)   (2)

In the expression (2), a second correction coefficient K2! is used toconvert the amount of calculated intake air Qsm_(n) ! as the amount ofexhaust gas flowing into the catalytic carrier into an amount of heatgiven to the catalyst by the exhaust gas at this time. The coefficientK2! takes account of the temperature of the exhaust gas assumed on thebasis of a current engine operating condition determined by a currentamount of intake air Q_(n) !, a current engine speed Ne_(n) !, a currenttemperature of the engine cooling water Thw_(n) !, and the like. A thirdcorrection coefficient K3! converts the amount of basic reaction heatBTr_(n) ! at this time which takes account of the degree of activationof the catalyst into an amount of actual reaction heat. The coefficientK3! takes account of the amount of exhaust gas and the air-fuel ratiodetermined on the basis of the current engine operating condition. Afourth correction coefficient K4! converts the amount of basic radiationheat BTo_(n) ! at this time which takes account of the temperature ofthe catalyst into an amount of actual radiation heat. The coefficientK4! takes account of the temperature of the atmosphere Ta_(n) ! and theamount of exhaust gas. Thus, the second coefficient K2!, the thirdcoefficient K3!, and the fourth coefficient K4! are again determined bythe use of maps (not shown) when the process at step 111 is repeated. Onthe other hand, a first correction coefficient K1! converts an amount ofheat which increases of decrease in such manner in the catalyst into anaverage varying value of the temperature in each portion of thecatalytic carrier 2.

Next, the routine goes to step 112 and an assumed temperature of thecatalyst at this time Tc_(n) ! is calculated in a manner that theaverage varying value of the temperature of the catalyst dTc_(n) ! isadded to the assumed temperature of the catalyst at the last timeTc_(n-1) !. Next, at step 113, it is determined if the above mentionedair-fuel ratio feed-back control F/B! is carried out. When the result isaffirmative, the routine goes to step 114, and a first integration valueLox1! of output local length of the first air-fuel ratio sensor 3 and asecond integration value Lox2! of output local length of the secondair-fuel ratio sensor 4 are read in from a routine shown in FIG. 6. Onthe other hand, when the result at step 113 is negative, for example, inthe case that a fuel-cut is carried out during a deceleration of theengine, the routine goes to steps 128, and the first integration valueLox1! is reset to 0! in the present routine and in the routine shown inFIG. 6. Next, at step 129, the second integration value Lox2! is resetto 0! in the present routine and in the routine shown in FIG. 6. Next,the routine is stopped.

Here, the routine shown in FIG. 6 is explained as follows. The routineis started simultaneously with the engine starting and is repeated atevery predetermined period dt! which is between one-tenth andone-thousandth of the repeating period of the first routine. Initially,at step 10, an output ox1_(n) ! of the first air-fuel ratio sensor 3 atthis time and an output ox2_(n) ! of the second air-fuel ratio sensor 4at this time are read in. At step 20, a difference d1! between theoutput ox1_(n) ! at this time and the output ox1_(n-1) ! at the lasttime of the first air-fuel ratio sensor 3, and a difference d2! betweenthe output ox2_(n) ! at this time and the output ox2_(n-1) ! at lasttime of the second air-fuel ratio sensor 4 are calculated.

Next, the routine goes to step 30, and the first output local lengthlox1! of the first air-fuel ratio sensor 3 and the second output locallength lox2! of the second air-fuel ratio sensor 4 are calculated byexpressions (3) and (4).

    lox1=(d1.sup.2 +dt.sup.2).sup.0.5                          (3)

    lox2=(d2.sup.2 +dt.sup.2).sup.0.5                          (4)

Next, at step 40, as shown in expressions (5) and (6), the first outputlocal length lox1! and the second output local length lox2! areintegrated respectively after previous integration values had been resetto 0!, and thus the first integration value Lox1! and the secondintegration value Lox2! are calculated. Thereafter, the routine isstopped.

    Lox1=Lox1+lox1                                             (5)

    Lox2=Lox2+lox2                                             (6)

Returning to the first routine. After the first and second integrationvalues Lox1! and Lox2!, which are calculated in such manner, are readin, the routine goes to step 115 and a ratio Lr_(n) ! of the firstintegration value Lox1! to the second integration value Lox2! iscalculated. The O₂ storage ability of the three-way catalytic converter1 drops according to the deterioration of the catalyst so that a currentO₂ storage ability is almost equivalent to a current purificationability of the catalyst. When the O₂ storage ability does not drop, theair-fuel ratio in exhaust gas downstream the catalyst becomes almoststoichiometric so that the second integration value Lox2! becomes aminimum and thus the ratio Lr_(n) ! becomes large. Conversely, when theO₂ storage ability is completely lost, the excess or deficiency ofoxygen in the exhaust gas is not completely compensated so that theoutput of the second air-fuel ratio sensor 4 varies the almost same asthe first air-fuel ratio sensor 3, and thus the second integration valueLox2! becomes a maximum. Therefore, the ratio Lr_(n) ! becomes 1!.Accordingly, the ratio Lr_(n) ! becomes equivalent to a currentpurification ability of the catalyst.

Next, the routine goes to step 116, and a threshold B_(n) ! of thepurification ability which corresponds the abnormal degree ofdeterioration of the catalyst in the current catalyst temperature isdetermined from a map shown in FIG. 7, on the basis of the assumedtemperature of the catalyst Tc_(n) ! at step 112.

The routine goes to step 117, and a difference D! between the thresholdB_(n) ! determined at step 116 and the current purification ability ofthe catalyst determined at step 115 is calculated and the routine goesto step 118. At step 118, it is determined if a flag f! is 1!. The flagf! is also reset to 0! when the engine is stopped and when the routinegoes to step 125. Accordingly, the routine goes to step 119 and it isdetermined if the difference D! is equal to or smaller than 0!. When theresult is affirmative, it is determined that the degree of deteriorationof the catalyst is normal. Next, at step 128, the first integrationvalue Lox1! is reset to 0!. At step 129, the second integration valueLox2! is reset to 0!. The routine is stopped.

On the other hand, when the result at step 119 is negative, the degreeof deterioration of the catalyst may reach an abnormal area. The routinegoes to step 120 and the flag f! is made 1!. Next, at step 121, anappraisal value at this time m_(n) ! is determined by a map shown inFIG. 8, on the basis of the difference D!. In the map, an appraisalvalue is set such that the larger the difference D! becomes, the largerthe appraisal value is.

Next, at step 122, an integration value M! of the appraisal value m_(n)! is calculated. The integration value M! is reset to 0! when the engineis stopped and when the routine goes to step 126. Thereafter, theroutine goes to step 123, and it is determined if the integration valueM! becomes larger than a predetermined plus value C1!. When the resultis negative, the routine goes to step 124 and it is determined if theintegration value M! has become smaller than a predetermined minus valueC2!. When the result is negative, the routine goes to step 128 and thefirst integration value Lox1! is reset to 0!. Next, at step 129, thesecond integration value Lox2! is reset to 0!. The routine is stopped.

When the flag f! is made 1!, in the next process through the routine,the result at step 118 is negative and thus the process after step 121is repeated. If during the process, the difference D! calculated at step117 is kept to become plus, i.e., the purification ability Lr_(n) !calculated at step 115 is kept to be smaller than the threshold B_(n) !determined at step 116, the integration value M! becomes large.Therefore, the result at step 123 is affirmative and at step 127, it isdetermined that the degree of deterioration of the catalyst is abnormaland this is informed to the driver by an alarm lamp 10.

The appraisal value m_(n) ! becomes minus when the difference D! becomesminus, i.e., when the purification ability Lr_(n) ! is larger than thethreshold B_(n) !. Accordingly, in case that the purification abilityLr_(n) ! is temporarily less than the threshold B_(n) ! in spite of thenormal degree of deterioration of the catalyst, the integration value M!becomes small while the process after step 121 is repeated. Therefore,the result at step 124 is negative and the routine goes to step 125 andthe flag f! is reset to 0!. At step 126, the integration value M! isreset to 0!. At step 128, the first integration value Lox1! is reset to0!. At step 129, the second integration value Lox2! is reset to 0!. Theroutine is stopped.

The premise of the present routine is that when the engine is started,the temperature of the catalyst is nearly equal to the temperature ofthe atmosphere Ta!, and assumes the temperature of the catalyst Tc!.Accordingly, when the result at step 104 is affirmative, i.e., when theengine is started again immediately after it was stopped, the flag F! ismade 1! and thereafter the result at step 103 remains affirmative andthus the routine is stopped without determining the degree ofdeterioration of the catalyst. If the result at step 104 is negativewhen the engine is started, the flag F! is made 2! and thereafter theresult at step 102 remains affirmative and thus the process after step108 is repeated.

Thus, according to the present routine, it is also determined if thedegree of deterioration of the catalyst is abnormal before the catalysthas been completely activated. Therefore, in case that the purificationability of the catalyst drops excessively only before it has beencompletely activated, it is also determined if the degree ofdeterioration of the catalyst is abnormal. Accordingly, in this case,the driver can be urged to exchange the catalyst for a new one and thusit is prevented that the exhaust gas is not purified sufficiently beforethe catalyst has been completely activated. In the determination, anassumed current purification ability is compared with a thresholdaccording to the temperature of the catalyst. In particular, apurification ability before the catalyst has been completely activatedis unstable, because it varies largely according to not only thetemperature thereof but also the harmful material content of the exhaustgas and the like. Accordingly, the above mentioned comparison of onlyone time can make a mistake in the determination. However, according tothe present routine, when the degree of deterioration of the catalystmay be abnormal, the appraisal value is determined every comparison, andit is determined that the degree of deterioration of the catalyst isabnormal when the integration value of the appraisal valueficationexceeds the predetermined value, so that the reliability ofdetermination can be improved considerably.

According to the present routine, in the map for determining anappraisal value shown in FIG. 8, an appraisal value m! is set such thatthe larger the difference D! becomes, the very much larger m! becomes.However, this does not limit the present invention. An appraisal valuem! may be set such the integration value M! becomes large at least whenthe difference D! becomes positive, for example, such that it is directproportion to the difference D! as shown in FIG. 9. According to thepresent routine, a threshold is determined at step 116, on the basis ofthe current temperature of the catalyst, and a difference between thethreshold and the current purification ability is calculated at step117, and an appraisal value is determined at step 121 on the basis ofthe difference. Of course, according to this idea, an appraisal value ispreset in a map shown in FIG. 13, in accordance with a temperature ofthe catalyst and a purification ability thereof, and thus steps 116 and117 are eliminated and at step 121 an appraisal value may be determinedby this map.

FIG. 10 shows a second routine for determining the abnormal degree ofdeterioration of the catalyst. The routine is started simultaneouslywith the engine starting and is repeated at every predetermined period.First, at step 201, a count value n! is increased by 1!, which value isreset to 0! when the engine is stopped. Next, the routine goes to step202 and it is determined if the above mentioned air-fuel ratio feed-backcontrol F/B! is carried out. When the result is negative, for example,in the case that a fuel-cut is carried out in a deceleration of theengine, the routine goes to step 209 and the first and secondintegration value Lox1!, Lox2! are reset to 0! in the present routineand in the routine shown in FIG. 6 since a current purification abilityof the catalyst can not be determined by the first and second air-fuelratio sensors 3, 4, as mentioned above. Next, the routine is stopped.

When the result at step 202 is affirmative, the routine goes to step 203and it is determined if the count value n! is equal to or larger than afirst predetermined value n1!. When the result is negative, i.e.,immediately after the engine is started, the routine goes to step 209and the first and second integration value Lox1!, Lox2! are reset to 0!in the present routine and in the routine shown in FIG. 6. Next, theroutine is stopped.

When the result at step 203 is affirmative, i.e., when a predeterminedperiod lapsed after the engine had been started so that the catalyst isduring or after the warming-up thereof, the routine goes to step 204 andthe first and second integration values Lox1!, Lox2! calculated in theroutine shown in FIG. 6 are read in. Next, at step 205, a currentpurification ability Lr_(n) ! is calculated as explained in the firstroutine.

Next, the routine goes to step 206 and it is determined if the countvalue n! is equal to or larger than a second predetermined value n2!.The result is negative immediately after the count value n! has exceededthe first predetermined value n1!, and thus the routine goes to step 207and another count value nb! is increased by 1!, which value is reset to0! when the engine is stopped. Next, at step 208, an integration valueSLrb! of the purification ability Lr_(n) ! when the count value n! isbetween the first and second predetermined values n1!, n2! iscalculated. Next, the above-mentioned process at step 209 is carried outand the routine is stopped.

Once such a process is repeated, the count value n! becomes larger thanthe second predetermined value n2!, i.e., a period lapsed after theengine starting is increased and the catalyst has been completely warmedup. At this time, the result at step 206 is affirmative and the routinegoes to step 210. At step 210, a further another count value na! isincreased by 1!. At step 211, an integration value SLra! of thepurification ability Lr_(n) ! when the count value n! is larger than thesecond predetermined value n2! is calculated.

Next, the routine goes to step 212 and it is determined if the countvalue na! is equal to the count value nb! increased at step 207. Thedetermination means that if an integration times of the purificationability after the catalyst has been completely warmed up, i.e., in acomplete activation condition, becomes equal to the integration times ofthe purification ability while the catalyst is warmed up, i.e., in anincomplete activation condition. When the result is negative, theabove-mentioned process at step 209 is carried out and the routine isstopped.

When the result at step 212 is affirmative, the routine goes to step 213and the integration value SLrb! is multiplied by a first coefficientk1!, the integration value SLra! is multiplied by a second coefficientk2!, and the sum of these multiplied values is made an overallpurification ability of the catalyst LR!. Here, the first coefficientk1! is smaller than the second coefficient k2!.

Next, the routine goes to step 214 and it is determined if the overallpurification ability LR! is larger than a threshold a! thereof whichcorresponds to the abnormal degree of deterioration of the catalyst.When the result is affirmative, the routine goes to step 215 and it isdetermined that the degree of deterioration of the catalyst is normal.On the other hand, when the result at step 214 is negative, the routinegoes to step 216 and it is determined that the degree of deteriorationof the catalyst is abnormal.

Thus, according to the present routine, the overall purification abilityLR! is determined to treat the integration value of the purificationability in an incomplete activation condition of the catalyst lightlyand one in a complete activation condition thereof heavily, by the firstand second coefficients k1!, k2!. Because, the integration value of thepurification ability in an incomplete activation condition of thecatalyst is unstable, and one in a complete activation condition thereofis stable and reliable. The overall purification ability LR! representsan actual purification ability accurately and takes account of thepurification ability in an incomplete activation condition of thecatalyst. Therefore, if the overall purification ability is used in thedetermination of the abnormal degree of deterioration of the catalyst,the determination can be made accurate in case of the usualdeterioration of the catalyst and in the case that the purificationability of the catalyst drops only in incomplete activation condition.

FIGS. 11 and 12 show a third routine for determining the abnormal degreeof deterioration of the catalyst. The differences between the secondroutine and the third routine only are explained as follows. In thisroutine, at step 312, it is determined if the count value na! is equalto the count value nb!. When the result is affirmative, the routine goesto step 313, and a first average value SLrb/nb! is calculated such thatthe integration value SLrb! in an incomplete activation condition of thecatalyst is divided by the count value nb!, and it is determined if thefirst average value is larger than a threshold b! thereof whichcorresponds to the abnormal degree of deterioration of the catalyst inan incomplete activation condition thereof.

When the result is affirmative, the routine goes to step 314 and asecond average value SLra/na! is calculated such that the integrationvalue SLra! in complete activation condition of the catalyst is dividedby the count value na!, and it is determined if the second average valueis larger than a threshold c! thereof which corresponds to the abnormaldegree of deterioration of the catalyst in a complete activationcondition thereof. When the result is affirmative, the routine goes tostep 315 and it is determined if a determination flag E! is 0!, whichflag is explained in detail as follows. The determination flag E! is setto 0! when the catalyst is new. When both of the results at steps 313,314 are affirmative, the determination flag E! is kept 0! so that theresult at step 315 is affirmative. The routine goes to step 316 and itis determined that the degree of deterioration of the catalyst isnormal.

On the other hand, when the result at step 313 is negative, i.e., whenthe first average value SLrb/nb! is smaller than the threshold b!, thedegree of deterioration of the catalyst may be abnormal so that theroutine goes to step 317 and the determination flag E! is increased by1!. Thereafter, the routine goes to step 314. If the result at step 314is affirmative, the routine goes to step 315. The result at step 315 isnegative and the routine goes to step 319.

At step 319, it is determined if the determination flag E! is equal toor larger than 2!. Now, the determination flag E! is 1! so that theresult is negative and the routine goes to step 320 and thus it isdetermined that the degree of deterioration of the catalyst isprovisional abnormal. Conversely, in the case that the degree ofdeterioration of the catalyst may be abnormal in a complete activationcondition thereof and is normal in an incomplete activation conditionthereof, the routine goes from step 313 through steps 314, 318, 319 tostep 320. It is also determined that the degree of deterioration of thecatalyst is provisionally abnormal.

On the other hand, when both of the results at steps 313 and 314 arenegative, i.e., when the degree of deterioration of the catalyst may beabnormal in incomplete and complete activation conditions, thedetermination flag E! is increased by 1! at steps 317, 318 so that theresult at step 319 is affirmative and the routine goes to step 321 andthus it is determined that the degree of deterioration of the catalystis abnormal.

In the next process of the routine, if the degree of deterioration ofthe catalyst is still provisionally abnormal, the determination flag E!is also increased by 1! at step 317 or 318 so that the result at step319 is affirmative and thus it is determined that the degree ofdeterioration of the catalyst is abnormal.

Thus, according to the present routine, when the purification ability ofthe catalyst detected in complete activation condition thereof is lessthan the threshold thereof, and the purification ability of the catalystdetected in an incomplete activation condition thereof is less than thethreshold thereof, it is sure that the catalyst deteriorates excessivelyso that it is determined that the degree of deterioration of thecatalyst is abnormal so that the driver is urged to exchange thecatalyst for a new one. When only one of the purification abilities ofthe catalyst detected in complete and incomplete activation conditionsthereof is less than the threshold thereof, the actual purificationability in a complete or an incomplete activation condition may dropexcessively or the detected purification ability may be accurate so thatit is determined that the degree of deterioration of the catalyst isprovisionally abnormal. It is unusual that only one of the purificationabilities in a complete and an incomplete activation conditions dropsexcessively. However, when this is repeated, it is sure that only one ofthe purification abilities drops excessively so that it is determinedthat degree of deterioration of the catalyst is abnormal.

In particular, the purification ability detected in an incompleteactivation condition can not be accurate since it is unstable. However,in this case, the present routine is not mistaken that degree ofdeterioration of the catalyst is abnormal.

As the method of detecting the purification ability of the catalyst,usual other methods, for example, a comparison between an inversionperiod (from lean side to rich side, or from rich side to lean side) ofoutput of the first air-fuel ratio sensor and of the second air-fuelratio sensor, or the comparison between a time integration value ofoutput of the first air-fuel ratio sensor and of the second air-fuelratio sensor, can be utilized.

In the above-mentioned three routines, as a current purification abilityof the catalyst, a current O₂ storage ability thereof is utilized.However, a current purification ability of the catalyst can be directlydetected such that at least one HC sensor detects HC concentration inthe exhaust gas. FIG. 14 is a sectional view of a part of an internalcombustion engine exhaust system with a device for determining theabnormal degree of deterioration of a catalyst, using two HC sensors.The difference between FIG. 14 and FIG. 1 is to have a first HC sensor 5and a second HC sensor 6, instead of the first air-fuel ratio sensor 3and the second air-fuel ratio sensor 4. The first and second HC sensors5, 6 produce an output voltage which is proportional to HC concentrationin the exhaust gas. The device 10' determines if the degree ofdeterioration of the catalyst is abnormal, according to a fourth routineshown in FIGS. 15 and 16.

The differences between the present routine and the above-mentionedfirst routine only are explained as follows. In this routine, when theair-fuel ratio feed-back control F/B! is carried out after the currenttemperature of the catalyst Tc_(n) ! is calculated at step 412, theroutine goes to step 415 and a current purification ability of thecatalyst AB_(n) ! is read in. The current purification ability AB_(n) !is calculated in a routine shown in FIG. 17. Thereafter, the routinegoes to step 416 and a threshold B_(n) '! of the purification abilitywhich corresponds the abnormal degree of deterioration of the catalystin the current temperature of the catalyst is determined by a map whichis similar to the map shown in FIG. 7. Next, at step 417, a differenceD'! between the threshold B_(n) '! and the purification ability AB_(n) !is calculated and the process after step 418 as same as the processafter step 118 in the first routine is carried out. In the case that thedegree of deterioration of the catalyst may be abnormal, when theintegration value of each appraisal value determined on the basis ofeach difference D'! in complete and incomplete activation conditions ofthe catalyst exceeds a predetermined value, it is determined that thedegree of deterioration of the catalyst is abnormal.

The routine shown in FIG. 17 is repeated at every predetermined periodas same as the repeating period of the fourth routine. The integrationof value on the basis of output of the sensor is not required so thatthe reset corresponding to steps 128, 129 in the first routine iseliminated. In this routine, at step 100, an output of the first HCsensor 5, i.e., HC concentration in exhaust gas flowing into thecatalytic carrier 2 HCSF!, and an output of the second HC sensor 6,i.e., HC concentration in exhaust gas flowing out from the catalyticcarrier 2 HCSR!, are read in. Next, at step 200, a rate of purificationof hydrocarbon HCP! is calculated by an expression (7).

    HCP=(1-HCSR/HCSF)                                          (7)

The rate of purification of hydrocarbon HCP! varies in accordance withnot only the purification ability of the catalyst but also a catalyticcarrier space speed (a ratio of an amount of exhaust gas to a catalyticcarrier capacity). Because, in case that the catalytic carrier spacespeed is large, if the purification ability of the catalyst is high, anamount of hydrocarbon blowing through the catalytic carrier withoutbeing purified becomes large. Accordingly, at step 300, a currentcatalytic carrier space speed SVR! is calculated such that a currentamount of intake air QSm_(n) ! calculated at step 410, as a currentamount of exhaust gas, is divided by the catalytic carrier capacityVol!. Next, at step 400, a fifth coefficient K5! is determined by a mapshown in FIG. 18, on the basis of the current catalytic carrier spacespeed SVR!, and a current purification ability of the catalyst AB_(n) !is calculated such that the rate of purification of hydrocarbon ismultiplied by the fifth coefficient K5!. In the map shown in FIG. 18,the fifth coefficient is set such that the rate of purification ofhydrocarbon drops when the catalytic carrier space speed SVR! is largerthan a predetermined value.

In the fourth routine, an appraisal value can be directly determined bythe map shown in FIG. 13, instead of steps 416, 417, as explained in thefirst routine.

The device 10' can determine if the degree of deterioration of thecatalyst is abnormal according to a fifth routine shown in FIG. 19,instead of the fourth routine. The differences between theabove-mentioned second routine and the fifth routine only are explainedas follows. In the fifth routine, at step 505, a current purificationability of the catalyst AB_(n) ! is read in from the routine shown inFIG. 17, the as same as in fourth routine. The purification ability isutilized instead of the purification ability on the basis of the O₂storage ability of the catalyst. Thereafter, at step 508, an integrationvalue SABb! of the purification ability in an incomplete activationcondition thereof is calculated. At step 511, an integration value SABa!of the purification ability in a complete activation condition thereofis calculated. At step 513, the integration value SABb! is multiplied bythe first coefficient k1!, the integration value SABa! is multiplied bythe second coefficient k2!, and the sum of these multiplied values ismade an overall purification ability of the catalyst ABR!, as explainedin the second routine. At step 514, when the overall purificationability ABR! exceeds a threshold a'! thereof, it is determined that thedegree of deterioration of the catalyst is abnormal.

The device 10' can determine if the degree of deterioration of thecatalyst is abnormal according to a sixth routine shown in FIGS. 20 and21, instead of the fourth routine. The differences between theabove-mentioned third routine and the sixth routine only are explainedas follows. In the sixth routine, at step 605, a current purificationability of the catalyst AB_(n) ! is read in from the routine shown inFIG. 17, as same as in fourth routine. The purification ability isutilized instead of the purification ability on the basis of the O₂storage ability of the catalyst. Thereafter, at step 613, it isdetermined if a first average value SABb/nb! in an incomplete activationcondition of the catalyst is larger than a threshold b'! thereof. Atstep 614, it is determined if a first average value SABa/na! in acomplete activation condition of the catalyst is larger than a thresholdc'! thereof. When both of these results are negative, it is determinedthat degree of deterioration of the catalyst is abnormal at step 621.When only one of these results is negative, it is determined that thedegree of deterioration of the catalyst is provisionally abnormal atstep 620. When this is repeated, it is determined that the degree ofdeterioration of the catalyst is abnormal at step 621.

In the fourth, fifth, and sixth routines, the purification ability ofthe catalyst is directly grasped by the HC sensors, instead ofindirectly grasping it by the air-fuel ratio sensors. Therefore, theobtained purification ability is more accurate so that the determinationof the abnormal degree of deterioration of the catalyst can be made moreaccurate. Moreover, these routines can determine if the degree ofdeterioration of the catalyst is abnormal in the catalytic converterwhich does not have the O₂ storage ability.

FIG. 22 is a sectional view of a part of an internal combustion engineexhaust system with a device for determining the abnormal degree ofdeterioration of a catalyst, using only one HC sensor. The differencebetween FIG. 22 and FIG. 14 is to have a HC sensor 7 downstream of thecatalytic carrier 2, instead of the first HC sensor 5 and the second HCsensor 6. The device 10" calculates the purification ability of thecatalyst according to a routine shown in FIG. 23, to determine if thedegree of deterioration of the catalyst is abnormal. The routine wouldbe explained as follows.

First, at step 1000, an output of the HC sensor 7, i.e., the HCconcentration in the exhaust gas flowing out from the catalytic carrier2 HCSR! is read in. Next, at step 2000, the HC concentration in theexhaust gas flowing into the catalytic carrier 2 KHCSF! is determined bya map shown in FIG. 24, on the basis of the current engine speed. Atstep 3000, a rate of purification of hydrocarbon HCP'! is calculated byan expression (8).

    HCP'=(1-HCSR/KHCSF)                                        (8)

Next, at step 4000, a current catalytic carrier space speed SVR! iscalculated as same as in the routine shown in FIG. 17. At step 5000, acurrent purification ability of the catalyst AB_(n) '! is calculatedsuch that the rate of purification of hydrocarbon is multiplied by thefifth coefficient K5!. The calculated purification ability of thecatalyst can be used in the fourth, fifth, or sixth routine. Thus, itcan be determined if the degree of deterioration of the catalyst isabnormal by only one HC sensor.

In the second, third, fifth, and sixth routines, each purificationability of the catalyst in the complete and the incomplete activationconditions thereof is calculated on the basis of many detected values.However, this does not limit the present invention. If only one detectedvalue in an incomplete activation condition of the catalyst is used asthe purification ability thereof, and only one detected value in acomplete activation condition of the catalyst is used as thepurification ability thereof, the reliability of determination of theabnormal degree of deterioration of the catalyst becomes higher than theprior art.

In the first and fourth routines, the current temperature of thecatalyst is calculated. Of course, a measured value of the catalysttemperature may be used, in stead of the calculated value.

In the first, second, and third routines, the current O₂ storage abilityis calculated as the current purification ability. In case that thecatalytic converter space speed is large, and if the O₂ storage abilityof the catalyst is high, an amount of oxygen blowing through thecatalytic carrier without being stored become large. Accordingly, if inthe calculation of the O₂ storage ability, a current catalytic carrierspace speed is taken account of, the grasped O₂ storage ability of thecatalyst, i.e., the grasped purification ability thereof becomes moreaccurate.

Although the invention has been described with reference to specificembodiments thereof, it should be apparent that numerous modificationscan be made thereto by those skilled in the art, without departing fromthe basic concept and scope of the invention.

We claim:
 1. A device for determining the abnormal degree ofdeterioration of catalyst of a catalytic converter arranged in aninternal combustion engine exhaust system, comprising:first purificationability grasping means for grasping a first purification ability of saidcatalyst in complete activation condition of said catalyst; secondpurification ability grasping means for grasping a second purificationability of said catalyst in incomplete activation condition of saidcatalyst; overall purification ability calculating means for calculatingan overall purification ability of said catalyst such that said firstpurification ability given a first weight is added to said secondpurification ability given a second weight which is less than said firstweight; and abnormality determining means for determining if the degreeof deterioration of said catalyst is abnormal by the comparison betweensaid overall purification ability and a predetermined threshold thereof.2. A device according to claim 1, wherein said first purificationability grasping means grasps said first purification ability, on thebasis of many purification abilities grasped in a complete activationcondition of said catalyst.
 3. A device according to claim 1, whereinsaid second purification ability grasping means grasps said secondpurification ability, on the basis of many purification abilitiesgrasped in an incomplete activation condition of said catalyst.
 4. Adevice according to claim 1, wherein said first and second purificationability grasping means have a detecting means for detecting an O₂storage ability of said catalyst, and grasp said first and secondpurification abilities of said catalyst on the basis of said O₂ storageability.
 5. A device according to claim 1, wherein said first and secondpurification ability grasping means have a detecting means for detectinga HC purification ability of said catalyst, and grasp said first andsecond purification abilities of said catalyst on the basis of said HCpurification ability.